Move from informal chats to a structured process. Set out clearly, in writing, what good looks like and exactly where they are falling short. Give them a genuine, supported chance to improve, with clear goals, a fair timescale and real help, and keep a short written note of each conversation.
If performance still does not improve after a fair process, you may be able to move toward a capability dismissal, but only once you can show the standards were clear, the chance to improve was real, and the process was fair.
Every situation has its own details, and those details change the right answer. For a read on yours, take the free Employee Situation Check or book a call.
General guidance to help you decide your next step, not legal advice. Employment law changes, so always check the current position for your own situation.